The present invention relates to alumina with its surface coated with diamond by means of vapor-phase growth methods. The thus coated alumina is usable for a cutting tool, a vibration plate of a speaker, a wear resistance part, etc., and further usable for an IC substrate, etc. since the diamond itself has very high hardness and a low dielectric constant and high thermal conductivity.
As is well known, a diamond is a crystal of carbon (C). Graphite has a stable crystal structure of C under atmosphere-pressure, and the crystal structure of a diamond is in an energy state higher than that of graphite. Generally, C-atoms provide a stable crystal structure as graphite under the condition of low pressure while as diamond under the condition of high pressure.
Some energy must be supplied to transform graphite into diamond. As means for supplying energy to the graphite, required is a condition of high temperature and high pressure at least 1,450.degree. C. and 55,000 atoms.
Further, it is general to use a catalyst so as to accelerate the transformation of graphite into diamond, and a metal alloy such as nickel (Ni) or cobalt (Co) has been used as the catalyst.
Nowadays, diamonds for industrial use have been produced by means of superhigh pressure as described above, and many of the thus obtained diamonds have been embedded in bits of lathes, in tip ends of drills, and the like, so as to be used as cutting tools.
Generally, it has been a common sense that superhigh pressure and a high temperature are indispensable for synthesizing diamond. However, recently, it has become possible to synthesize diamond thin films from gasses under low pressure and low temperature by means of a chemical vapor phase growth method or the like, that is, a CVD (Chemical Vapor Deposition) method, in which a gas is decomposed to grow a solid, without using superhigh pressure, and it is considered that such diamond thin flims are applicable to various technical fields.
Conventionally, the condition for forming a diamond film with good crystallinity by means of a CVD method is such that a hydrocarbon gas diluted with a hydrogen gas having a volume of more than ten times as large as that of the former is excited by plasma or a heated tungsten filament and a substrate is heated to about 700.degree.-1,000.degree. C., as described in Japanese Patent Application Laid-Open Nos. 135117/83, 110494/83 and 3098/84, etc.
If a diamond film is formed on a surface of a metal material, it is possible to increase the wear resistance of the material and to reduce a friction coefficient of the same. Since the temperature for synthesizing a diamond film by means of the abovementioned CVD method is generally higher than 700.degree. C., there is such a problem that materials for substrate on which diamond films are formed are limited. On the other hand, if a diamond film is formed, for example, on a ceramic material, it is possible to give properties such as improved wear resistance and improved corrosion resistance to the substrates material, which have never been provided to it.
In the prior art references disclosing such CVD methods for synthesizing diamond films, Japanese Patent Application Laid-Open No. 106478/86 is relevant to the present application. This prior art discloses a product coated with diamond, which consists of a substrate of ceramic sintered body, a first layer of carbide, nitride, oxide, boride, etc. of periodic table 4a, 5a or 6a group metals on the substrate and a second layer of silicon nitride, silicon carbide, etc., on the first layer, and further coated diamond and/or diamond-like carbon on the second layer. Also, Japanese Patent Application Laid-Open No. 161897/86 discloses a vibrator of a speaker consisted of an alumina layer directly coated with diamond or diamond-like carbon.
However, if a surface of alumina is coated simply with a diamond film by means of the above-mentioned CVD method, the diamond film is apt to peel off so that no useful diamond coating has been realized. It is considered that such peeling is caused by a high synthesizing temperature (generally, more than 700.degree. C.) of the diamond film which generates thermal distortion and large residual stress in the film.
The above mentioned Japanese Patent Application Laid-Open No. 106478/86 discloses a variety of ceramic sintered materials provided with first and second layers and further diamond coating layer. Although it is certain that the coating layers of such a structure are useful, the more the number of the layer is, the more the number of manufacturing steps is. Accordingly, the thus obtained product becomes expensive. Furthermore, when plural coating layers are formed, thickness of each coating layer other than a diamond film becomes thick so that the effects of diamond itself are decreased.